This information was taken from the Geology Tours compiled by Allan Donaldson & Chris Findlay - Geotour of Frontenac Arch Biosphere Reserve (2008) and Geology Boat Tour of the St Lawrence, Thousand Islands 2010 booklet.
The Creation of the Frontenac Arch
The geological story of the Frontenac Arch region could begin at the plate tectonic merging of all the Earth’s landmasses to form the supercontinent Rodinia about 1100 million years ago. It was only up until two billion years ago did the core of North America, Laurentia, take form (brown core on figure below). The ancient continent of Laurentia at the time was positioned fairly close to the equator and would eventually become the core of the North American continent. Between about 1.3 billion years and 1 billion years ago, a second large continental mass moved northwesterly against Laurentia, crushing its margins like a giant vice. As a result of the collision, the ancient (1.8 to 2.5 billion years) Precambrian rocks that formed the edge of Laurentia were shoved down into the crust, deformed by heat and pressure, and then thrust up to form the northeasterly-trending Laurentian Mountain Belt. The roots of this former mountain belt, known geologically as the Grenville Province, extend from the coast of Labrador, across Quebec, Ontario and northern New York and then disappear beneath younger rocks of central U.S.A. A northwesterly-trending relic of this belt that connects Grenville rocks of the Algonquin Highlands in Ontario with similar rocks of the Adirondack Highlands in New York State is known as the Frontenac Arch.
Precambrian Rock Profile of the Frontenac Arch
The Frontenac Arch,asdescribed earlier was once a rugged mountain belt (the Laurentian Mountain Belt) that had been eroded to a relatively uniform Precambrian plain with a maximum relief generally less than 100 meters. Beginning about 500 million years ago, the Cambrian and Ordovician sedimentary rocks (sandstone, limestone, shale) that form the cover over the Precambrian rocks were deposited in shallow seas that flooded the region. Whether these rocks originally covered the Arch itself and were later stripped away by erosion and ice or whether the Arch was a positive feature at that time (i.e. stood above water level) and did not receive sediments is an open question, although the evidence seems to favour the latter.
The rocks forming the Frontenac Arch, called the Grenville Series or Grenville Supergroup, are exposed along a southwest-trending belt that stretches from the present day coast of Labrador through Quebec and Ontario and then disappears under younger rocks of central U.S.A. They are a complex of sedimentary, volcanic and igneous rocks that have been converted by heat and pressure to a sequence of metamorphic rocks that include various gneisses (e.g. granitic gneiss, quartz-biotite-feldspar gneiss), migmatites (layered gneisses that have been invaded by narrow granitic bands parallel to the layering), quartzite and crystalline limestone. The part of the Grenville Supergroup that underlies the Thousand Island region is called the Central Metasedimentary Belt or CMB. As the name suggests, it is a belt of metamorphosed sediments representing a series of crustal fragments (now “Terranes” or Domains”) thrust to the northwest as the belt grew over time
At any rate, the older Grenville rocks (granite, granite gneiss, schist, quartzite, marble) were relatively resistant to erosion and commonly form prominent knolls and humps where the original cover of younger sediments was either stripped away or never deposited. The present day St. Lawrence River, flowing over these Precambrian knobs and knolls (modified locally by glacial ice scouring and polishing), has given rise to the topography of the Thousand Islands.
The Ice Age – Pleistocene (Late Quaternary) Glaciation
About 20,000 years ago, much of eastern Canada and northeast U.S.A. was covered by the great Laurentide Ice Sheet (several kilometers thick). The eastern part of the ice sheet, known as the Labrador lobe, flowed southerly from its center in Ungava. In the Frontenac Arch region, ice moved southwesterly, roughly parallel to the geological “grain” of the Grenville rocks. Between about 17,000 and 14,000 years BP (before present) the ice began to melt back to the north. By about 12,500 years BP a large glacial lake called Lake Iroquois had formed to the west of the Arch, forerunner of the smaller Lake Ontario. A few hundred years later, the ice had melted back to allow seawater of the Champlain Sea to flood most of the area of the present Gulf of St. Lawrence, the Ottawa and St. Lawrence river valleys and Lake Champlain. During this period, marine clays and silts of the Champlain Sea were deposited on top of earlier glacial sediments. As the sea retreated, these deposits (glacial sediments and marine clays) were reworked by wave action in shallow waters, resulting in a mélange (mixture) of deposits, whose ancestry is locally problematic. By about 9,000 years BP the Champlain Sea had disappeared and by about 6,000 years BP, the present Great Lakes and Ottawa and St. Lawrence river drainage patterns had been established.
The above figure reflects the landscape about 9,000 years ago – near the end of the last ice age. The arm of the Atlantic Ocean reached into the Frontenac Arch from the melting of the continental ice sheet before the significant rebounding of the crust that had been depressed beneath the glacial ice for thousands of years.